Just in case there's people around still having this kind of issue, this is what fixed it for me.

Go to your "process settings" (with "Show Advanced" enabled) -> then "Other" -> check the "Dimensional Adjustments" settings.
That needs to be set to 0.00 mm. In my case it was set to 0.50 mm and that's exactly the error I was having, about 1mm of error all around the models.

Of course you may want to set it to just to a lower value (like 0.05 or 0.01) but that's what fixed it for me.

Had a similar problem which I proposed a change to overcome. But I don´t think anyone understod me. Anyway, one could add the ability to just offset the inner outlines. Maybe it does not work to code, do not know, but it would open many possibilities.viewtopic.php?f=23&t=3083&p=13334#p13334